Electric machines in motor vehicles can be used as what is generally known as starter generators. On the one hand, their task consists of starting the internal combustion engine during the operation of the electric machine in the motor mode; on the other hand, it consists of generating current for the vehicle electrical system and of charging the battery of the motor vehicle during the operation of the electric machine in the generator mode. Such electric machines are able to be connected to the internal combustion engine or to the crankshaft via a belt, e.g., via the ribbed V-belt of the internal combustion engine (known as belt-driven starter generators, BSG).
In most cases, special properties of the belt, such as its length, are precisely adapted to the system that is made up of a starter generator, an internal combustion engine and possibly additional components connected to the belt, such as ventilators or coolant pumps. Wear and an elongation of the belt may lead to a restricted function of the system, e.g., to decreasing preloading, and thus to a decreasing transmittable torque.
A method for the detection of slip in the drive mechanism of starter generator systems is described in German Patent Application No. DE 101 12 568 A1. Slip is detected at a comparison stage, which is supplied with input variables that are dependent upon the rate of rotation, especially a rate-of-rotation of a three-phase current generator and a crankshaft speed.